Optical film and polarizing beam splitter

ABSTRACT

Optical films and polarizing beam splitters including the optical films are described. In some cases, the optical film includes a first optical stack disposed on, and spaced apart by one or more spacer layers from, a second optical stack, each optical stack comprising a plurality of polymeric interference layers reflecting and transmitting light primarily by optical interference in a same predetermined wavelength range. Each optical stack has interference layers closer to the one or more spacer layers that reflect longer wavelengths and interference layers farther from the one or more spacer layers that reflect shorter wavelengths.

BACKGROUND

A polarizing beam splitter may include a reflective polarizer disposed between the hypotenuses of adjacent prisms. The reflective polarizer may be a multilayer polymeric film.

SUMMARY

In some aspects of the present description, an optical film including a plurality of polymeric interference layers is provided. Each interference layer reflects or transmits light primarily by optical interference for at least one wavelength in a predetermined wavelength range extending at least from 450 nm to 1000 nm. A total number of the interference layers is greater than about 100 and less than about 300. For light in the predetermined wavelength range and substantially normally incident on the optical film, the plurality of interference layers has an average optical transmittance greater than about 85% for a first polarization state, an average optical reflectance greater than about 95% for an orthogonal second polarization state, and an average optical transmittance less than about 5% for the second polarization state.

In some aspects of the present description, an optical film including a plurality of stacked first polymeric interference layers disposed on a plurality of stacked second polymeric interference layers is provided. Each first and second interference layers reflects or transmits light primarily by optical interference for at least one wavelength in a same predetermined wavelength range. For an outermost first interference layer being the first interference layer farthest from the plurality of stacked second interference layers and an outermost second interference layer being the second interference layer farthest from the plurality of stacked first interference layers, the outermost first and second interference layers have optical thicknesses equal to one quarter of respective first and second wavelengths in the predetermined wavelength range. A difference between the first and second wavelengths is less than about 40 nm.

In some aspects of the present description, an optical film including a first optical stack disposed on, and spaced apart by one or more spacer layers from, a second optical stack is provided. Each optical stack includes a plurality of polymeric interference layers reflecting and transmitting light primarily by optical interference in a same predetermined wavelength range extending at least from 450 to 600 nm, such that for light in the predetermined wavelength range and substantially normally incident on the optical film, the plurality of the interference layers in each optical stack transmits at least 80% of light having a first polarization state, reflects at least 90% of light having an orthogonal second polarization state, and transmits less than 5% of light having the second polarization state. Each spacer layer in the one or more spacer layers does not reflect or transmit light primarily by optical interference. Each optical stack has interference layers closer to the one or more spacer layers that reflect longer wavelengths and interference layers farther from the one or more spacer layers that reflect shorter wavelengths. The first and second optical stacks and the one or more spacer layers are formed integrally with one another.

In some aspects of the present description, an optical film transmitting at least 80% of normally incident light having a first polarization state in a predetermined wavelength range and reflecting at least 95% of normally incident light having an orthogonal second polarization state in the predetermined wavelength range is provided. The optical film comprises a plurality of polymeric layers where each polymeric layer has an average thickness less than about 200 nm. The plurality of polymeric layers includes first and second polymeric layers as the two polymeric layers in the plurality of polymeric layers farthest apart from each other, the first and second layers having respective first and second thicknesses, a difference between the first and second thicknesses being less than about 10 nm.

In some aspects of the present description, an optical film having opposing first and second major surfaces and including adjacent non-overlapping first and second optical stacks disposed therebetween is provided. The first optical stack is disposed closer to the first major surface and farther from the second major surface and the second optical stack is disposed closer to the second major surface and farther from the first major surface. For each optical stack and the major surface closest to the optical stack: the optical stack includes a plurality of first interference layers numbering between 50 and 300; each first interference layer reflects or transmits light primarily by optical interference; each first interference layer closer to the major surface is thinner than each first interference layer farther from the major surface, each first interference layer has orthogonal in-plane indices of refraction nx and ny and an index of refraction nz in a thickness direction of the first interference layer, a difference between ny and nz being less than 0.008 and a difference between nx and ny being greater than 0.2. The first optical stack is integrally formed with the second optical stack.

In some aspects of the present description, a polarizing beam splitter (PBS) including a first prism having a first hypotenuse; a second prism having a second hypotenuse facing the first hypotenuse; and an optical film disposed between and adhered to the first and second hypotenuses is provided. The optical film includes a plurality of alternating higher index and lower index layers disposed between opposing first and second major surfaces of the optical film. The optical film substantially transmits light having a first polarization state and substantially reflects light having an orthogonal second polarization state in a predetermined wavelength range extending from 400 nm to 700 nm. For a collimated light having the second polarization state and incident on the optical film within the PBS after going through an optical lens having an f-number between 1.8 to 2.2 and centered on an optical axis making an angle of about 30 to 60 degrees with the optical film, the optical film has a total transmittance T1 when the light is first incident on the first major surface of the optical film and a total transmittance T2 when the light is first incident on the second major surface of the optical film, where a maximum difference between T1 and T2 is less than 0.02% as a function of a wavelength of the incident light in the predetermined wavelength range.

In some aspects of the present description, a polarizing beam splitter (PBS) including a first prism having a first hypotenuse; a second prism having a second hypotenuse facing the first hypotenuse; and an optical film disposed between and adhered to the first and second hypotenuses is provided. The optical film includes a plurality of alternating higher index and lower index layers disposed between opposing first and second major surfaces of the optical film. The optical film substantially transmits light having a first polarization state and substantially reflects light having an orthogonal second polarization state in a predetermined wavelength range extending from 400 nm to 700 nm. When the PBS is incorporated in an imaging system including a point source, a collimating optical lens for collimating light emitted by the point source, and an imaging optical lens having an f-number in a range of 4.5 to 5.5, with the optical film in the PBS receiving light from the imaging optical lens and reflecting the received light toward an image surface, the imaging system has a point spread function having a full width at 20% of maximum less than 33 micrometers.

In some aspects of the present description, a polarizing beam splitter (PBS) including a first prism having a first hypotenuse; a second prism having a second hypotenuse facing the first hypotenuse; and an optical film disposed between and adhered to the first and second hypotenuses is provided. The optical film includes a plurality of alternating higher index and lower index layers disposed between opposing first and second major surfaces of the optical film. The optical film substantially transmits light having a first polarization state and substantially reflects light having an orthogonal second polarization state in a predetermined wavelength range extending from 400 nm to 700 nm. When the PBS is incorporated in an imaging system including a point source, a collimating optical lens for collimating light emitted by the point source, and an imaging optical lens having an f-number in a range of 4.5 to 5.5, with the optical film in the PBS receiving light from the imaging optical lens and reflecting the received light toward an image surface, the imaging system has a point spread function having a full width at 15% of maximum less than 60 micrometers.

In some aspects of the present description, a polarizing beam splitter (PBS) including a first prism having a first hypotenuse; a second prism having a second hypotenuse facing the first hypotenuse; and an optical film disposed between and adhered to the first and second hypotenuses is provided. The optical film includes a first optical stack disposed on, and spaced apart by a light absorbing linear polarizer from, a second optical stack. The first optical stack is closer to the first hypotenuse and farther from the second hypotenuse, and the second optical stack is closer to the second hypotenuse and farther from the first hypotenuse. For each optical stack and the hypotenuse closest to the optical stack: the optical stack includes a plurality of interference layers numbering between 50 and 300 and reflecting and transmitting light primarily by optical interference in a same predetermined wavelength range extending at least from 400 to 600 nm, the interference layers closer to the hypotenuse being configured to primarily reflect shorter wavelengths in the predetermined wavelength range and the interference layers farther from the hypotenuse being configured to primarily reflect longer wavelengths in the predetermined wavelength range. When the PBS is incorporated in an imaging system where an image light entering the PBS exits the PBS after being sequentially reflected by the first optical stack, transmitted by the optical film, and reflected by the second optical stack, the light absorbing linear polarizer absorbs at least 50% of image light scattered by at least one of the first and second optical stacks while absorbing less than 2% of the image light.

In some aspects of the present description, a polarizing beam splitter (PBS) including a first prism having a first hypotenuse; a second prism having a second hypotenuse facing the first hypotenuse; and an optical film disposed between and adhered to the first and second hypotenuses is provided. The optical film includes a plurality of alternating higher index and lower index layers disposed between opposing first and second major surfaces of the optical film. The optical film substantially transmits light having a first polarization state and substantially reflects light having an orthogonal second polarization state in a predetermined wavelength range extending at least from 430 nm to 630 nm. For a collimated light having the first polarization state and incident on the optical film within the PBS after going through an optical lens having an f-number between 1.8 to 2.2 and centered on an optical axis making an angle of about 30 to 60 degrees with the optical film, the optical film has a total transmittance T3 when the light is first incident on the first major surface of the optical film and a total transmittance T4 when the light is first incident on the second major surface of the optical film. An average over the predetermined wavelength range of each of T3 and T4 is at least 92%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic perspective view of a multilayer optical film;

FIG. 1B is a schematic perspective view of a portion of the multilayer optical film of FIG. 1A;

FIG. 2 is a schematic side view of an optical film which includes first and second optical stacks;

FIG. 3 is a schematic illustration of the layer thickness profile of an optical film;

FIGS. 4-5 are schematic side views of polarizing beam splitters;

FIGS. 6-8 are schematic cross-sectional views of optical systems;

FIG. 9 is a schematic graph of a point spread function of an imaging system;

FIG. 10 is a schematic plot of a transmittance of an optical film in a polarizing beam splitter for s-polarized light as a function of wavelength;

FIG. 11 is a schematic plot of a transmittance of an optical film in a polarizing beam splitter for p-polarized light as a function of wavelength;

FIG. 12 is a plot of the thickness of sequentially optical repeat units (ORUs) in a reflective polarizer film as a function of the ORU number in the film;

FIG. 13 is a plot of the transmittance of an optical film in a polarizing beam splitter for s-polarized light as a function of wavelength;

FIG. 14 is a plot of the transmittance of an optical film in a polarizing beam splitter for p-polarized light as a function of wavelength;

FIG. 15-18 are plots of the transmittance of optical films in polarizing beam splitters for s-polarized light as a function of wavelength;

FIG. 19 is a plot of the transmittance of an optical film in a polarizing beam splitter for p-polarized light as a function of wavelength;

FIG. 20 is a plot of the point spread function of an imaging system utilizing a polarizing beam splitter including a reflective polarizer film; and

FIGS. 21-22 are plots of transmission coefficients for reflective polarizer films.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanying drawings that form a part hereof and in which various embodiments are shown by way of illustration. The drawings are not necessarily to scale. It is to be understood that other embodiments are contemplated and may be made without departing from the scope or spirit of the present description. The following detailed description, therefore, is not to be taken in a limiting sense.

According to some embodiments of the present description, optical films are provided which have improved optical properties over conventional films. In some embodiments, the optical films are reflective polarizer films suitable for use in a polarizing beam splitter which is suitable for use in various optical systems including head-mounted virtual reality or augmented reality displays. In some embodiments, the optical films include a packet of alternating polymeric interference layers having a high reflectivity (e.g., greater than about 95%, 96%, or 97%) over a predetermined wavelength range (e.g., 400 to 600 nm, or 450 to 600 nm or 430 nm to 680 nm, or 430 to 630 nm, or 450 nm to 1000 nm) with a limited total number of interference layers (e.g., less than about 300). In some embodiments, a high reflectivity over a broad band (e.g., over at least 400 nm or 450 nm to at least 1000 nm or 1050 nm) is achieved with a limited total number of interference layers (e.g., less than about 300). In some cases, a higher long wavelength band edge (e.g., at least 1000 nm or at least 1050) is desired for improved performance when the film is used in a polarizing beam splitter. In some embodiments, the optical films include two (or more) packets of alternating polymeric interference layers with the thicker interference layers of the first packet closer to the thicker interference layers of the second packet and the thinner interference layers of the first packet farther from the thinner interference layer of the second packet. It has been found that such optical films can simultaneously provide a high-quality image reflection and a very low transmission leak (e.g., a transmission in the block state of less than 0.05%). In some embodiments, a dichroic polarizer (light absorbing linear polarizer) is included between the two packets. It has been found that this can reduce image contamination from multiple reflections between the two packets and/or can mitigate effects of de-polarizing scattering within the optical film. In some embodiments, the optical films of the present description give an improved point spread function (e.g., a narrower full width at 20% of maximum) when used in a polarizing beam splitter of an imaging system compared to an otherwise equivalent imaging system utilizing conventional optical films.

The optical films described herein may be characterized as a multilayer optical film having plurality of optical layers (e.g., interference layers) configured to selectively transmit and reflect light within a predetermined wavelength range. In some such embodiments, the optical films function as a reflective polarizer that selectively transmits and reflects light of different polarization states. For example, FIG. 1A is a schematic perspective view of an example of a multilayer optical film 100 that includes a plurality interference layers 102 positioned along a central axis to form optical film 100 having a total of (N) interference layers 102. FIG. 1B is a schematic perspective diagram of a segment of the optical film 100 illustrating alternating interference layers 102 a and 102 b. FIGS. 1A-1B include a coordinate system that defines x, y, and z directions.

During use, light incident on a major surface of optical film 100 (e.g., film surface 104), depicted by incident light 110 may enter a first layer of optical film 100 and propagate through the plurality of interference layers 102, undergoing select reflection or transmission by optical interference depending on the polarization state of incident light 110. Incident light 110 may include a first polarization state (a) and a second polarization state (b) that are be mutually orthogonal to one another. The first polarization state (a) may be considered as the “pass” state while the second polarization state (b) may be considered as the “reflected” or “block” state. As incident light 110 propagates through plurality of interference layers 102, portions of the light in the second polarization state (b) will be reflected by adjacent interference layers resulting in the second polarization state (b) being reflected by optical film 100, while a portion of the light in the first polarization state (a) collectively passes through optical film 100.

In some embodiments, the optical film 100 may be characterized in terms of its reflectivity and transmissivity of the first and second polarization states (a) and (b) of incident light 110. For example, the amount of incident light 110 for a predetermined wavelength transmitted through optical film 100 may be expressed as the percent of optical transmittance (Ta) for the first polarization state (a) and the percent of optical transmittance (Tb) for the second polarization state (b). The amount of incident light 110 for a predetermined wavelength range reflected by optical film 100 may be expressed as the percent of optical reflectance (Ra) for the first polarization state (a) and the percent of optical reflectance (Rb) for the second polarization state (b). For a given optical film, the sum of transmissivity, reflectivity, and losses due to, for example, absorption, will amount to 100% for light within a predetermined wavelength range. In some embodiments, the optical film 100 has a relatively low absorbance for light within the predetermined wavelength range. In some embodiments, the relatively low absorbance of incident light 110 by optical film 100 may result less heat generated within optical film 100 and leading to an overall more efficient reflective film. In other embodiments, the optical film 100 may include dichroic absorbing layers as described further elsewhere herein.

The predetermined wavelength range may be any suitable wavelength range, including for example, visible light (e.g., about 400-700 nm), a range of visible light (e.g., about 400 nm, or about 420 nm, or about 430 nm, or about 450 nm to about 600 nm, or about 630 nm, or about 635 nm, or about 650 nm, or about 680 nm, or about 700 nm), near-infrared (e.g., about 800-1300 nm), a range based on the output of a light source such as a liquid crystal display backlight (e.g., 425-675 nm), and a range based on providing a desired bandwidth at off normal incidence (e.g., 400 nm or 450 nm to 1000 nm or to 1050 nm). In some embodiments, optical film 100 may be configured to transmit and reflect light of different polarizations states within more than one predetermined wavelength range, e.g., visible light and near-infrared. For example, the predetermined wavelength range may include a first range from about 430 nm to about 465 nm, a second range from about 490 nm to about 555 nm, and a third range from about 600 nm to about 665 nm. In some embodiments, optical film 100 may include multiple stack/packets, as described further elsewhere herein, that each include a plurality of interference layers, where each stack/packet may be directed to a different predetermined wavelength range or may be directed to a same predetermined wavelength range. In preferred embodiments, each stack/packet is configured to reflect a substantially same predetermined wavelength range.

In some embodiments, the interference layers may be characterized as a series of two-layer unit cells or optical repeat units. The thickness of each unit cell may be configured to reflect a target wavelength within the predetermined wavelength range. In some examples, the central wavelength of reflectivity for a unit cell corresponds to the twice the optical thickness of a two-layer unit cell. Therefore, to reflect a predetermined wavelength range (e.g. 420 to 635 nm), the unit cells within the stacks/packets will have different thicknesses to cover wavelengths from the left band-edge to the right band-edge. The optical thickness of a layer refers to the index of refraction of the layer times the physical thickness of the layer. In the case of an optical film configured to reflect light polarized along a block axis and transmit light polarized along an orthogonal pass axis, the index of refraction used in determining the optical thickness is the index of refraction along the block axis. The two layers in the optical repeat units may have equal or approximately equal optical thicknesses. In some cases, it is useful to characterize the optical repeat unit in terms of an “f-ratio” which is the optical thickness of the higher index layer in the pair of layers divided by the total optical thickness of the layer pair. In some embodiments, the f-ratio is about 0.5. An f-ratio of 0.5 may be preferable since this maximizes the reflective power of the 1^(st) order (primary) reflection band of an optical stack or packet of interference layers.

In some embodiments, optical film 100 includes less than about 1200 (N) interference layers 102, where each interference layer 102 reflects or transmits incident light 110 primarily by optical interference. In some embodiments, optical film 100 includes less than about 1000, or less than about 800, or less than about 600, or less than about 300 interference layers 102. While 1200 or more interference layers 102 may be included in optical film 100, in some cases, it may be desirable to achieve the desired optical performance using fewer total layers in order to reduce the overall thickness of the film, since reducing the overall thickness of a display assembly (e.g., LCD displays) is preferable in many applications. Additionally or alternatively, the fewer total number of interference layers 102 may reduce the complexity in of the manufacturing process as well as reduce the potential for introducing variability (e.g., spectral variability in block or pass states) or production errors (e.g., increased block state transmission due to depolarization between the layers, reduced pass state transmission) in the final optical film. In some embodiments, the total number N of interference layers 102 is greater than about 50, or greater than about 100, or greater than about 150, or greater than about 200.

In some embodiments, the optical film 100, or an optical stack included in the optical film 100, has an average optical transmittance (Ta) greater than about 85% for a first polarization state (a), an average optical reflectance (Rb) greater than about 95% for an orthogonal second polarization state (b), and an average optical transmittance (Tb) less than about 5% for the second polarization state (b) for a substantially normally incident light 110 in a predetermined wavelength range. In some embodiments, Ta is greater than about 80%, or greater than about 85%, or greater than about 87% or greater than about 89%. In some embodiments, Rb is greater than about 90%, or greater than about 95%, or greater than about 96%, or greater than about 97%, or greater than about 98%. In some embodiments, Tb is less than about 5%, or less than about 4%, or less than about 3%, or less than about 2%, or less than about 1%, or less than about 0.5%, or less than about 0.3%, or less than about 0.2%, or less than about 0.1%, or less than about 0.05%, or less than about 0.04%, or less than about 0.03%, or less than about 0.02%, or less than about 0.01%. In some embodiments, a desired Ta, Tb and Rb is achieved using greater than about 50, or greater than about 100 and less than about 1200, or less than about 600 or less than about 300 total interference layers 102 in the optical film 100 or in an optical stack included in the optical film 100. A high Rb (e.g., greater than about 95%) and a low Tb (e.g., less than about 5%) with a relatively small number of layers can be achieved by choosing materials for the interference layers and controlling the stretch ratios of the film so that the refractive index difference between adjacent interference layers for light having the second polarization state is small (e.g., less than 0.008) and the refractive index difference between adjacent interference layers for light with the first polarization state is large (e.g., greater than about 0.2). The refractive index or index of refraction can be taken to be the index at a wavelength of 550 nm if no wavelength is specified.

The transmittance of an optical film refers generally to the transmitted light intensity divided by the incident light intensity (for light of a given wavelength, incident direction, etc.), but may be expressed in terms of “external transmittance” or “internal transmittance”. The external transmittance of an optical film is the transmittance of the optical film when immersed in air, and without making any corrections for Fresnel reflections at the air/element interface at the front of the element or for Fresnel reflections at the element/air interface at the back of the element. The internal transmittance of an optical film is the transmittance of the film when the Fresnel reflections at its front and back surfaces have been removed. The removal of the front and back Fresnel reflections may be done either computationally (e.g. by subtracting an appropriate function from the external transmission spectrum), or experimentally. For many types of polymer and glass materials, the Fresnel reflections are about 4 to 6% (for normal or near-normal angles of incidence) at each of the two outer surfaces, which results in a downward shift of about 10% for the external transmittance relative to the internal transmittance. If transmittance is referred to herein without being specified as internal or external, it may be assumed that the transmittance refers to external transmittance, unless otherwise indicated by the context.

The reflectance and transmittance of an optical film when used between two prisms in a polarizing beam splitter (PBS), for example, may be expressed in terms of the total reflectance and total transmittance, respectively, of the optical film as used in the PBS. The total transmittance is the transmitted power in a given wavelength divided by the incident power within the prism in which the light at the given wavelength is incident. Similarly, the total reflectance is the reflected power in a given wavelength divided by the incident power within the prism in which the light at the given wavelength is incident.

In some embodiments, interference layers 102 of optical film 100 includes alternating layers (e.g., A and B depicted in FIG. 1B) of two different polymeric materials that exhibit differing index of refraction characteristics. As shown in FIG. 1B, optical film 100 includes of alternating layers (e.g., ABABA . . . ) of different optical materials referred to as material “(A)” and material “(B)”. As described further elsewhere herein, the various layers of the two different materials may be formed through an extrusion/lamination process in which the layers are extruded together to form the multiple optical layers 102 (ABABA . . . ) that are adhered together.

In some embodiments, during the extrusion process the optical layers 102 may be stretched to impart the various interference characteristics of the film. For example, layers of the A and B optical material may be stretched (e.g., in a 5:1 ratio or a 6:1 ratio) along one axis (e.g., the X-axis), and not stretched appreciably along the orthogonal axis (e.g., the Y-axis). The indices of refraction along the X, Y and Z axes are denoted nx, ny, nz, respectively, for either the A or B layers. The indices of refraction may also be denoted as n1x, n1y, n1z and n2x, n2y, n2z for the A and B layers, respectively, along the along the X, Y and Z axes, respectively.

The selection of optical material used to form the A and B layers may be selected to impart specific optical characteristics to the film as a result of the stretching process. For example, the (B) material forming optical layers 102 b may have a nominal index of refraction (e.g., n2 between 1.5 and 1.6) which is not substantially altered by the stretching process. As such, the index of refraction for “B” layers 102 b in both the x and y directions (n2x and n2y) may be substantially the same for both directions after the stretching process and may be substantially the same as the index of refraction in the thickness direction (n2z). In contrast, the (A) material forming optical layers 102 a may have an index of refraction altered by the stretching process. For example, a uniaxially stretched layer 102 a of the (A) material may have a higher index of refraction in the X-axis or stretched direction 120 (e.g., 1.8≤n1x≤1.9), and a different index of refraction associated with the Y-axis or non-stretched direction 122 (e.g., 1.5≤n1y≤1.6) which may substantially equal the index of refraction in the thickness direction (e.g., 1.5≤n1z≤1.6). In some embodiments, an absolute value of a difference between n1y and n1z is less than 0.008 and a difference between n1x and n1y is greater than about 0.2. Due to the increased index of refraction in the stretched direction, layers 102 a including material (A) may be considered as the high index of refraction (HIR) layers 102 a while interference layers 102 b including material (B) may be considered as the low index of refraction (LIR) layers 102 b. In some embodiments, an absolute value of a difference between n2y and n2z is less than 0.005. In some embodiments, each of n2x, n2y and n2z is between 1.5 and 1.6. In some embodiments, the refractive indices of the alternating AB layers may be may be controlled by judicious materials selection and processing conditions. In some embodiments, the optical characteristics of the layers 102 causes optical film 100 to act as a reflecting polarizer that will substantially transmit the first polarization state (a) component of incident light 110 within a predetermined wavelength range oriented with respect to the non-stretched axis 122, while the stretched axis 120, will correspond to the reflect-axis for which the component of incident light 110 in second polarization state (b) within the predetermined wavelength range will be substantially reflected through optical interference.

In some embodiments, optical film 100 may be characterized by the difference between the indices of refraction between alternating HIR layers 102 a and LIR layers 102 b along the stretched axis 120 (i.e., Δnx=n1x−n2x). In some such embodiments, the indices of refraction between alternating HIR layers 102 a and LIR layers 102 b along the non-stretched axis direction 122 may be substantially the same such that the difference between the indices in non-stretched axis direction 122 (i.e., Δny=n1y−n2y) is about 0.0 (e.g., |Δny| less than about 0.02, or less than about 0.01, or less than about 0.005). In some examples, increasing the Δnx between HIR and LIR layers 102 a, 102 b (e.g., via choice of materials and/or control of the uniaxial orientation of the film) may permit sufficient transmission/reflection of polarized light for a given wavelength range using a fewer total number of interference layers as compared to an optical film with a lower Δnx for with the same optical power. For example, each packet of the reflective polarizer film of Example 1 contained 276 interference layers (138 optical repeat units) and each packet had an average optical transmittance greater than 85% for normally incident light in air in the pass state, and an average optical reflectance greater than 95% and an average optical transmittance less than 5% for normally incident light in air in the bock state where the averages were over the wavelength range from about 400 nm or about 450 nm to about 1000 nm or about 1050 nm. (The data shown in FIG. 21 is for a two-packet polarizer, but since most of the reflection is due to the packet facing the incident light, similar results hold for a reflective polarizer having only one of the two packets). Thus, a suitable reflectance and transmittance was obtained with a similar number of layers as some conventional reflective polarizer films but over a significantly larger bandwidth.

Preferably, the stretched axis direction of each of interference layers 102 will be substantially aligned (e.g., aligned or nearly aligned) such that the X-axis for each respective layer 102 represents the direction for obtaining the maximum index of refraction within the X-Y plane for each layer. However due to machine tolerances and number of interference layers 102, the stretched axis 120 for each of the interference layers (e.g., representing the direction of obtaining the maximum index or refraction for the layer) may be aligned to within a variance of about ±2°.

In some embodiments, optical film 100, or an optical stack included in optical film 100, may include a total of more than 100 and less than 600 (N) or less than 300 (N) first layers 102 a and second layers 102 b that reflect or transmit light primarily by optical interference. For example, optical film 100 may include less than 300 and greater than 100 first layers 102 a and less than 300 and greater than 100 second layers 102 b. As another example, optical film 100 may include between 50 and 300 first layers 102 a and may also include between 50 and 300 second layers 102 b. In some embodiments, for each pair of adjacent first and second layers 102 a, 102 b, the layers may define a stretched axis that represents the direction in which the maximum index of refraction obtained for the respective layer (e.g., X-axis/direction 120 corresponding to indices of refraction n1x and n2x for the two layers). The difference of indices of refraction between the first layer 102 a and second layer 102 b for the primary axis (e.g., Δnx=n1x−n2x) may be greater than about 0.2 or greater than about 0.24. In some such embodiments, the respective stretched axis directions for each of first and second optical layers 102 a, 102 b may be substantially aligned such that interference layers 102 define a maximum angular range of the respective stretched-axis directions of less than about 2 degrees.

Optical film 100 including plurality of interference layers 102 may be formed using any suitable technique. General techniques for forming multilayer optical films are described in U.S. Pat. No. 5,882,774 (Jonza et al.) “Optical Film”, U.S. Pat. No. 6,179,948 (Merrill et al.) “Optical Film and Process for Manufacture Thereof”, U.S. Pat. No. 6,783,349 (Neavin et al.) “Apparatus for Making Multilayer Optical Films”, and patent application publication US 2011/0272849 (Neavin et al.) “Feedblock for Manufacturing Multilayer Polymeric Films”. For example, layers 102 a and 102 b including optical materials A and B respectively may be fabricated using coextruding, casting, and orienting processes to form stacks/packets of tens to hundreds of interference layers 102, followed stretching or otherwise orienting the extruded layers to form a stack/packet of interference layers 102. Each stack/packet may include between about 200 and 1000 total interference layers depending on the desired characteristics of optical film 100. As used herein a “stack/packet” is used to refer to a continuous set of alternating interference layers 102 a, 102 b that is absent of any spacer or non-interference layers formed within the stack/packet (e.g., sequentially arranged). In some embodiments, spacer, non-interference layers, or other layers may be added to the outside of a given stack/packet, thereby forming the outer layers of the film without disrupting the alternating pattern of interference layers 102 within the stack/packet.

In some embodiments, optical film 100 may be fabricated by coextrusion. The fabrication method may comprise: (a) providing at least a first and a second stream of resin corresponding to the first and second polymers to be used in the finished film; (b) dividing the first and the second streams into a plurality of layers using a suitable feedblock, such as one that comprises: (i) a gradient plate comprising first and second flow channels, where the first channel has a cross-sectional area that changes from a first position to a second position along the flow channel, (ii) a feeder tube plate having a first plurality of conduits in fluid communication with the first flow channel and a second plurality of conduits in fluid communication with the second flow channel, each conduit feeding its own respective slot die, each conduit having a first end and a second end, the first end of the conduits being in fluid communication with the flow channels, and the second end of the conduits being in fluid communication with the slot die, and (iii) optionally, an axial rod heater located proximal to said conduits; (c) passing the composite stream through an extrusion die to form a multilayer web in which each layer is generally parallel to the major surface of adjacent layers; and (d) casting the multilayer web onto a chill roll, sometimes referred to as a casting wheel or casting drum, to form a cast multilayer film. This cast film may have the same number of layers as the finished film, but the layers of the cast film are typically much thicker than those of the finished film.

After cooling, the multilayer web can be re-heated and drawn or stretched to produce the near-finished multilayer optical film. The drawing or stretching accomplishes two goals: it thins the layers to their desired final thicknesses profile, and it orients the layers such that at least some of the layers become birefringent. The orientation or stretching can be accomplished along the cross-web direction (e.g. via a tenter), along the down-web direction (e.g. via a length orienter), or any combination thereof, whether simultaneously or sequentially. If stretched along only one direction, the stretch can be “unconstrained” (wherein the film is allowed to dimensionally relax in the in-plane direction perpendicular to the stretch direction) or “constrained” (wherein the film is constrained and thus not allowed to dimensionally relax in the in-plane direction perpendicular to the stretch direction). If stretched along both in-plane directions, the stretch can be symmetric, i.e., equal along the orthogonal in-plane directions, or asymmetric. Alternatively, the film may be stretched in a batch process. In any case, subsequent or concurrent draw reduction, stress or strain equilibration, heat setting, and other processing operations can also be applied to the film.

The polymers of the various layers are preferably chosen to have similar rheological properties, e.g., melt viscosities, so that they can be co-extruded without significant flow disturbances. Extrusion conditions may be chosen to adequately feed, melt, mix, and pump the respective polymers as feed streams or melt streams in a continuous and stable manner. Temperatures used to form and maintain each of the melt streams may be chosen to be within a range that avoids freezing, crystallization, or unduly high pressure drops at the low end of the temperature range, and that avoids material degradation at the high end of the range.

Example (A) materials suitable for optical film 100 may include, for example, polyethylene naphthalate (PEN), copolymers containing PEN and polyesters (e.g., polyethylene terephthalate (PET) or dibenzoic acid), glycol modified polyethylene terephthalate. Example (B) materials suitable for optical film 100 may include, for example, copolyesters based on PEN, copolyesters based on PET, polycarbonate (PC), or blends of these three classes of materials. To achieve high reflectivities with a reasonable number of layers, adjacent microlayers can exhibit a difference in refractive index (Δnx) for light polarized along the x-axis of at least 0.2, for example.

While optical film 100 may be described in some embodiments as having greater than about 100 and less than about 600 or less than about 300 total (N) interference layers 102, it will be appreciated that the lower bound of the total number of layers (N) may be any suitable amount configured to obtain the described optical properties. In some embodiments, there may be a trade-off between the obtained optical properties and the total number of layers (N)/thickness of the resultant film. For example, while in some embodiments the contrast ratio (Ta/Tb) of a film may generally increase by increasing the total number of interference layers 102 included in optical film 100 absent any manufacturing complications as discussed prior, the thickness of the film will also increase with the increasing number of layers. In some embodiments, such as in modern thin optical display devices, the overall thickness of a film may be a limiting factor as the availability for space in such optical display units is limited. In some embodiments, optical film 100 may provide a significant increase in one or more optical properties (e.g., contrast ratio), while having a significantly reduced film thickness (e.g., half) compared to other film constructions (e.g., the combined absorptions polarizer and reflective polarized used in some conventional display units). In addition, excessive thickness of the film may carry the risk of decreasing the overall contrast ratio due to depolarization of the pass-state light propagating through the film.

In some embodiments, optical film 100 may have between about 100 to about 1200 total interference layers 102 with an overall thickness for optical film 100 of less than about 100 μm including any optional non-interference or protective layers. In some embodiments, optical film 100 has a total thickness of less than about 100 μm (e.g., less than 80 μm or in a range of 50 μm to 80 μm) across all of the layers of optical film 100.

In some embodiments, the thickness of the individual interference layers 102 may be relatively thin such that fewer than 30% of interference layers 102 have a thickness greater than about 200 nm (e.g., less than 5% of interference layers 102 have a thickness greater than 200 nm or all interference layers 102 have a thickness less than about 200 nm), but may vary as function of position within optical film 100. In some examples, optical film 100 may be characterized in terms of the film's thickness profile. For example, the thickness of the individual interference layers 102 may vary such that the thickness of the individual interference layers 102 generally increases (e.g., increasing thickens apart from local fluctuations) moving from an outermost interference layer to an interference layer near a center of the optical film 100.

In some embodiments, an optical film includes more than one stack or packet of interference layers. FIG. 2 is a schematic side view of optical film 200 which includes first optical stack 202-1 and second optical stack 202-2 disposed between opposing first and second major surfaces 204 and 214 of the optical film 200. Each of the first and second optical stacks 202-1 and 202-2 include a plurality of interference layers and may be as described for the plurality of interference layers 102. The number of interference layers included in optical film 200 may be significantly larger than schematically illustrated in FIG. 2 and may be in any of the ranges described elsewhere herein. The first and second optical stacks 202-1 and 202-2 are non-overlapping in that they do not share any layers and not layers of one optical stack are interspersed with layers of the other optical stack. First and second protective boundary layers (PBLs) 205 a and 205 b are included at opposite sides of the first optical stack 202-1, and first and second protective boundary layers 207 a and 207 b are included at opposite sides of the second optical stack 202-2. In the illustrated embodiments, the PBLs 205 a and 207 a are outermost layers of the optical film 200. An intermediate layer 209 is included between PBLs 205 b and 207 b. The intermediate layer 209 together with the PBLs 205 b and 207 b may be described as spacer layers between the first and second optical stacks 202-1 and 202-2. Alternatively, the first and second optical stacks 202-1 and 202-2 may be described as including their respective PBLs, and the intermediate layer 209 may be described as a spacer layer. In some embodiments, each of the PBLs 205 a, 205 b, 207 a and 207 b are optically thick (i.e., a thickness substantially larger than a wavelength in the predetermined wavelength range). In some embodiments, an optically thick layer has a thickness greater than about 1 micrometer, or greater than 2 times a largest wavelength in the predetermined wavelength range, or greater than 3 times a largest wavelength in the predetermined wavelength range. In some embodiments, the intermediate layer 209 is optically thick.

In some embodiments, the intermediate layer 209 is a light absorbing linear polarizer. In some embodiments, the light absorbing linear polarizer has a block axis substantially aligned (e.g., within 5 degrees, or within 2 degrees) with each of the block axes (second polarization state) of the first and second optical stacks 202-1 and 202-2. In some embodiments, the intermediate layer 209 is a dichroic polarizer transmitting at least 80% of light having the first polarization state and absorbing at least 50% of light having the second polarization state. In some embodiments, optical film 200 is made by forming a melt stream including each layer of the optical film 200 which dichroic dyes in the intermediate layer 209, then casting the melt stream onto a chill roll, and then substantially uniaxially orienting the cast film. The high index layers of the first and second optical stacks 202-1 and 202-2 and the dichroic dyes in the intermediate layer may consequently be oriented along substantially the same direction.

In some embodiments, the optical film 200 is integrally formed. As used herein, a first element “integrally formed” with a second element means that the first and second elements are manufactured together rather than manufactured separately and then subsequently joined. Integrally formed includes manufacturing a first element followed by manufacturing the second element on the first element. An optical film including a plurality of layers is integrally formed if the layers are manufactured together (e.g., by casting from a common melt stream and then orienting the cast layers) rather than manufactured separately and then subsequently joined. Integrally formed optical films having more than one packet may be made, for example, by forming melts streams in separate packet creators and merging the packets into a common melt stream, then casting the common melt stream to form a cast film, and then orienting (e.g., by substantially uniaxially stretching) the cast film.

In some embodiments, each of the first and second optical stacks 202-1 and 202-2 includes a plurality of interference layers (e.g., polymeric interference layers) reflecting and transmitting light primarily by optical interference in a same predetermined wavelength range extending at least from 450 to 600 nm (e.g., 400 nm to 700 nm), or at least from 400 to 600 nm, or at least from 450 to 650 nm, or at least from 400 to 700 nm.

In some embodiments, optical film 200 transmits at least 80% of normally incident light having a first polarization state in a predetermined wavelength range and reflects at least 95% of normally incident light having an orthogonal second polarization state in the predetermined wavelength range. The optical film 200 may include a plurality of polymeric layers (the interference layers of the first and second optical stacks 202-1 and 202-2) where each polymeric layer has an average thickness less than about 200 nm. The plurality of polymeric layers may include first and second polymeric layers 203 a and 203 b as the two polymeric layers in the plurality of polymeric layers farthest apart from each other. The first and second layers 203 a and 203 b have respective first and second thicknesses (e.g., thicknesses t₁ and t_(N) depicted in FIG. 3). In some embodiments, an absolute value of a difference between the first and second thicknesses is less than about 20 nm, or less than about 10 nm, or less than about 8 nm, or less than about 7 nm.

In some embodiments, optical film 200 includes a plurality of stacked first polymeric interference layers (the interference layers of first optical stack 202-1) disposed on a plurality of stacked second interference layers (the interference layers of second optical stack 202-2), each first and second interference layers reflecting or transmitting light primarily by optical interference for at least one wavelength in a same predetermined wavelength range. In some embodiments, an outermost first interference layer (first layer 203 a) is the first interference layer farthest from the plurality of stacked second interference layers, an outermost second interference layer (second layer 203 b) is the second interference layer farthest from the plurality of stacked first interference layers. In some embodiments, the outermost first and second interference layers have optical thicknesses equal to one quarter of respective first and second wavelengths in the predetermined wavelength range. In some embodiments, a difference between the first and second wavelengths is less than about 80 nm, or less than about 60 nm, or less than about 40 nm, or less than about 30 nm, or less than about 20 nm, or less than about 10 nm. In some embodiments, each of the first and second wavelengths is in a range from about 420 nm to about 480 nm.

In some embodiments, an innermost first interference layer 206 a is the first interference layer closest to the plurality of stacked second interference layers, an innermost second interference layer 206 b is the second interference layer closest to the plurality of stacked first interference layers, and the innermost first and second interference layers 206 a and 206 b have optical thicknesses equal to one quarter of respective third and fourth wavelengths in the predetermined wavelength range. In some embodiments, a difference between the third and fourth wavelengths is less than about 120 nm, or less than about 100 nm, or less than about 80 nm, or less than about 60 nm, or less than about 40 nm, or less than about 30 nm, or less than about 20 nm, or less than about 10 nm. In some embodiments, each of the third and fourth wavelengths is in a range from about 600 nm to about 635 nm.

A light ray 213 reflecting from the second optical stack 202-2 is illustrated. Other light rays may pass through second optical stack 202-2 and be reflected by first stack 202-1. Some of the light reflected by first stack 202-1 may be absorbed by intermediate layer 209 and some of this light may be reflected by the second optical stack 202-2 or transmitted through the second optical stack 202-2. In some cases, light will be scattered by at least one of the first and second optical stacks. Scattered light generally propagates in directions other than the specular reflection direction and can result from impurities or defects in the optical film, for example. A light ray 215 at a higher incidence angle passes through the second optical stack 202-2 and is scattered from the first optical stack 202-1 and absorbed by intermediate layer 209. More generally, some light, particularly light at high incidence angles, can be scattered by at least one of the first and second optical stacks 202-1 and 202-2 and the intermediate layer 209 can absorb at least a portion of the scattered light.

A variety of layer thickness profiles can be used in the optical films of the present description. In some embodiments, the optical film includes two optical stacks/packets where each stack/packet has a thickness profile that substantially overlaps so that the two optical stacks/packets reflect a same predetermined wavelength range. In has been found that arranging the optical packets so that the thinner interference layers are closer to an outermost surface of the optical film and the thicker interference layers are further from either outermost surface gives improved optical properties in various applications (e.g., in display applications utilizing a polarizing beam splitter) and are thus typically preferred, though other configurations are possible. The optical films may provide one or more of a higher reflectivity for s-polarized light, a higher transmission of p-polarized light, and a lower transmission of s-polarized light over a wider range of incidence angles compared to conventional reflective polarizer films when used in a polarizing beam splitter in an optical system, for example.

FIG. 3 is a schematic illustration of the layer thickness profile of an optical film, such as optical film 200, which includes two optical stacks or packets. First optical stack 302-1 includes a plurality of interference layers having thicknesses ranging from t₁ at an outermost interference layer of the optical film to t_(m) at an outermost interference layer of the first optical stack 302-1, which is an interior interference layer of the optical film closest to the second optical stack 302-2. Second optical stack 302-2 includes a plurality of interference layers having thicknesses ranging from t_(m+1) at an outermost interference layer of the second optical stack 302-2, which is an interior interference layer of the optical film closest to the first optical stack 302-1, to t_(N) at an outermost interference layer of the optical film. In some embodiments, |t₁−t_(N)| is less than about 20 nm, or less than about 15 nm, or less than about 12 nm, or less than about 10 nm, or less than about 8 nm, or less than about 7 nm, or less than about 6 nm. In some embodiments, |t_(m)−t_(m+1)| is less than about 40 nm, or less than about 30 nm, or less than about 20 nm, or less than about 15 nm, or less than about 12 nm, or less than about 10 nm.

In some embodiments, an optical film includes a plurality of polymeric layers, each polymeric layer having an average thickness less than about 200 nm, the plurality of polymeric layers including first and second polymeric layers as the two polymeric layers in the plurality of polymeric layers farthest apart from each other (e.g., layers 203 a and 203 b), where the first and second layers having respective first and second thicknesses t₁ and t_(N), and a difference between the first and second thicknesses (differences between thicknesses being defined herein as non-negative) is less than about 20 nm or is in any of the ranges described for |t₁−t_(N)|. In some embodiments, the optical film includes first and second optical stacks spaced apart by one or more spacer layers, where the first optical stack includes a first plurality of layers in the plurality of polymeric layers, the second optical stack includes a second plurality of layers in the plurality of polymeric layers, the first optical stack includes the first polymeric layer, and the second optical stack includes the second polymeric layer. In some embodiments, the optical film includes a third polymeric layer in first plurality of layers (e.g., layer 206 a in the first optical stack 202-1) and a fourth polymeric layer in the second plurality of layers (e.g., layer 206 b in the second optical stack 202-2) as the two polymeric layers in the respective first and second pluralities of layers that are closest to each other, where the third and fourth layers have respective third and fourth thicknesses, and a difference between the third and fourth thicknesses is less than about 40 nm or is in any of the ranges described for |t_(m)−t_(m+1)|.

The optical films of the present description can be used in a variety of applications. For example, the optical films can be used in place of the reflective polarizer customarily used in display applications. In some embodiments, an optical film of the present description is incorporated into a polarizing beam splitter (PBS).

FIG. 4 is a schematic side view of a polarizing beam splitter (PBS) 450 including a first prism 430 and a second prism 440. First prism 430 includes a first hypotenuse 432 and second prism 440 includes a second hypotenuse 442. An optical film 400 is disposed between and adhered to the first and second hypotenuses 432 and 442. The optical film 400 may be adhered to the first and second hypotenuses 432 and 442 through respective optically clear adhesive layers (not illustrated in FIG. 4). The optical film 400 may be any of the optical films described herein. For example, optical film 400 may correspond to optical film 100 or 200 and may include a plurality of alternating higher index and lower index layers disposed between opposing first and second major surfaces of the optical film. In some embodiments, the optical film 400 corresponds to the optical film 200 with the first optical stack 202-1 closer to the first hypotenuse 432 and farther from the second hypotenuse 442, and the second optical stack 202-2 closer to the second hypotenuse 442 and farther from the first hypotenuse 432. In this case, layer 203 a is the interference layer closest to the first hypotenuse 432 and layer 203 b is the interference layer closest to the second hypotenuse 442.

The prism may have cross-sections that are right isosceles triangles as schematically illustrated in FIG. 4 or may have other cross-sections. For example, one of the sides of the prism may be curved or the cross-section may be a non-isosceles triangle. The prisms may have a length (into the page) about the same as the lateral dimensions or substantially different from the lateral dimensions. For example, the length may be substantially larger than the lateral dimensions so that the prism is substantially rod shaped. The rod may subsequently be singulated into multiple prisms which may be substantially cubic prisms.

FIG. 5 is a schematic side view of a polarizing beam splitter (PBS) 550 which is in many ways similar to PBS 450. PBS 550 includes a first prism 530 having a first hypotenuse 532, and a second prism 540 having a second hypotenuse 542 which faces the first hypotenuse 532. An optical film 500 is disposed between and adhered to the first and second hypotenuses 532 and 542 through first and second optically clear adhesive layers 534 and 544, respectively. The optical film 500 includes a first optical stack 502-1 disposed on, and spaced apart by a light absorbing linear polarizer 509 from, a second optical stack 502-2. The first optical stack 502-1 is closer to the first hypotenuse 532 and farther from the second hypotenuse 542, and the second optical stack 502-2 is closer to the second hypotenuse 542 and farther from the first hypotenuse 534. In some embodiments, for each optical stack and the hypotenuse closest to the optical stack: the optical stack includes a plurality of interference layers numbering between 50 and 300 and reflecting and transmitting light primarily by optical interference in a same predetermined wavelength range extending at least from 400 nm to 600 nm (for example, from 400 nm to 600 nm or from 400 nm to 700 nm), or extending at least from 450 nm to 600 nm, or extending at least from 450 nm to 650 nm, or extending at least from 450 nm to 1000 nm (e.g., from 450 nm to 1000 nm or from 450 nm to 1050 nm). In some embodiments, for each optical stack and the hypotenuse closest to the optical stack: the interference layers closer to the hypotenuse are configured to primarily reflect shorter wavelengths in the predetermined wavelength range and the interference layers farther from the hypotenuse are configured to primarily reflect longer wavelengths in the predetermined wavelength range. For example, the thickness profile of the interference layers in the optical film 500 may appear as in FIG. 3 with thinner layers (which reflect shorter wavelengths) closer to the outer surfaces of the optical film and thicker layers (which reflect longer wavelengths) farther from the outer surfaces of the optical film. In some embodiments, the PBS 550 has an optical axis 555 or is used in an optical system having the optical axis 555. In some embodiments, the optical axis 555 makes an angle α of about 30 to 60 degrees, or about 35 to 55 degrees, or about 40 to 50 degrees with the optical film 500.

The prisms used in the PBSs of the present description can be made from any suitable material. For example, the first and second prisms may, independently, be made from glass (e.g., BK7 glass) or polymeric materials such as polycarbonate or polyacrylate.

The polarizing beam splitters of the present description can be used in a variety of applications. For example, the PBSs may be used in an imaging or display system. The display system may be a head-mounted display such as a virtual reality display or an augmented reality display. The use of a PBS in various display applications is discussed, for example, in U.S. Pat. No. 8,382,293 (Phillips, III et al.), and U.S. Pat. No. 9,535,256 (Carls et al.). The PBSs of the present description can be used in place of the PBS of any of the optical systems described in these references. The PBSs of the present description are particularly advantageously used in optical systems where non-modulated light from a light source is reflected from one side of the optical film and a spatially modulated light from an imager is reflected from an opposite side of the optical film. Illustrative embodiments of such optical systems are illustrated in FIGS. 6-7.

FIG. 6 is a schematic cross-sectional view of an optical system 675 including a PBS 650, a light source 670, a pixelated spatial light modulator 672, and a reflective component 673. The PBS 650 may be any PBS of the present description and includes an optical film 600 disposed between first and second prisms 630 and 640. The light source 670 produces a light output 676 which is substantially polarized in a block state of the optical film 600. For example, light source 670 may include an unpolarized light source and a linear polarizer disposed to transmit light polarized along the y′-axis, referring to the x′, y′, z′ coordinate system depicted in FIG. 6, which may be the block state of the optical film 600. The pixelated spatial light modulator 672 may be any suitable spatial light modulator. For example, pixelated spatial light modulator 672 may be a liquid crystal on silicon (LCoS) panel with a quarter-wave retarder between the LCoS panel and the PBS 650. The reflective component 673 is preferably a polarization changing reflector. For example, reflective component 673 may include a minor and a quarter-wave retarder disposed between the mirror and the PBS 650. The light output 676 from the light source 670 is reflected from the optical film 600 and then reflected from the pixelated spatial light modulator 672 as an modulated light 678 having a pass polarization state and which is transmitted through the optical film 600, then reflected from reflective component 673 in the block polarization state, and then reflected from optical film 600. In some embodiments, light 676 is first incident on the optical film 600 in an s-polarization state which is a block polarization state of the optical film 600, then after reflecting from pixelated spatial light modulator 672 is incident on the optical film in a p-polarization state, which is a pass polarization state, and then after reflecting from reflective component 673 is again incident on the optical film 600 in an s-polarization polarization state.

In some embodiments, optical film 600 includes first and second optical stacks and optionally a light absorbing linear polarizer disposed therebetween. In some embodiments, the first optical stack faces the first prism 630 and substantially reflects light output 676 from light source 670, and the second optical stack faces the second prism 640 and substantially reflects light 678 incident on the optical film 600 from the reflective component 673.

FIG. 7 is a schematic cross-sectional view of an optical system 775 including a PBS 750, a light source 770, a pixelated spatial light modulator 772, and a reflective component 773. The PBS 750 may be any PBS of the present description and includes an optical film 700 disposed between first and second prisms 730 and 740. The light source 770 produces a light output 776 which is substantially polarized in a block state of the optical film 700 and may be as described for light source 670. The pixelated spatial light modulator 772 may be any suitable spatial light modulator and may be as described for pixelated spatial light modulator 672. The reflective component 773 is preferably a polarization changing reflector as described for reflective component 673. In some embodiments, reflective component 773 includes a quarter-wave retarder and a minor disposed on a surface of a lens which may be bonded to or may be spaced apart from the first prism 730. In other embodiments, the first prism 730 includes a curved outer surface and reflective component 773 is disposed on the curved outer surface of the first prism 730. The light output 776 from the light source 770 is reflected from the optical film 700 and then reflected from reflective component 773 in a pass polarization state, then transmitted through optical film 700, then reflected from the pixelated spatial light modulator 772 as a patterned light 778 having the block polarization state which is then reflected from optical film 700.

Either of the optical systems 675 or 775 may include further components (e.g., magnification optics and/or a waveguide) and may be used in a head-mounted display, for example. Optical system 675 and/or optical system 775 may be described as an imaging system.

In some embodiments, optical film 700 includes a first optical stack disposed on, and spaced apart by a light absorbing linear polarizer from, a second optical stack as described further elsewhere herein. The light output 776 may be referred to as an image light since it can be patterned by the pixelated spatial light modulator 772 to form an image. In some embodiments, the image light enters the PBS, and exits the PBS after being sequentially reflected by the first optical stack, transmitted by the optical film 700, and reflected by the second optical stack, where the light absorbing linear polarizer absorbs at least 50%, or at least 60%, or at least 70% of image light scattered by at least one of the first and second optical stacks while absorbing less than 2%, or less than 1.5%, or less than 1% of the image light. The pixelated spatial light modulator 772 may be fully on when determining the absorption of the absorbing linear polarizer so that the light output 776 is approximately the image light minus light absorbed by the light absorbing linear polarizer.

FIG. 8 is a schematic cross-sectional view of an optical system 875 which is useful for determining various properties of the PBSs and optical films described herein. Optical system 875 includes an unpolarized point light source 870, a linear absorbing polarizer 881, a collimating optical lens 883 for collimating light emitted by the point light source 870, an optical lens 885, a PBS 850 including first and second prisms 830 and 840 and an optical film 800 disposed between the first and second prisms 830 and 840 and receiving light 876 from the optical lens 885. The point light source 870 may include a light behind a screen with a pinhole (e.g., a hole with a small diameter—for example, 30 micrometers). A portion of the light 876 is reflected from optical film 800 to a detector 888 which is configured to determine the intensity of this reflected light and thereby determine the reflectance R. An x′, y′, z′ coordinate system is illustrated in FIG. 8. The linear absorbing polarizer 881 may be oriented to transmit light having an electric field along a specified direction (e.g., along the y′-axis or along the z′-axis) in the y′-z′ plane.

In some embodiments, the optical lens 885 is an imaging optical lens. In such embodiments, the optical system 875 may be referred to as an imaging system. In some embodiments, the detector 888 is disposed at an image surface 890 (surface where the optical system forms an image), and similarly, in some embodiments, the detector 889 is disposed at an image surface 892. In some embodiments, the optical lens 885 has an f-number in a range of 4.5 to 5.5, or in a range of 4.9 to 5.1, or the f-number may be nominally 5.0 but may differ from 5.0 (e.g., by less than about 1%) due to ordinary manufacturing variations, for example. In some embodiments, the detector 888 is configured to determine a point spread function (PSF) of the imaging system. The PSF of an imaging system describes the response (e.g., spread of light output at the image surface 890) of the imaging system to the point source. FIG. 9 schematically illustrates a PSF for an imaging system. The PSF has a maximum value of Max and has full width at half maximum (FWHM) 991, a full width at 30% of maximum (FW30% Max) 993, and a full width at 20% of maximum (FW20% Max) 995. The PSF can be normalized so that Max equals 1 without affecting the values of the various widths. In some embodiments, the imaging system has a point spread function having a full width at 20% of maximum less than 33 micrometers, or less than 32 micrometers, or less than 31 micrometers, or less than 30.5 micrometers, or less than 30 micrometers. In some embodiments, the imaging system has a point spread function having a full width at 20% of maximum greater than 15 micrometers, or greater than 20 micrometers. In some embodiments, the imaging system has a point spread function having a full width at 15% of maximum less than 62 micrometers, or less than 60 micrometers, or less than 58 micrometers, or less than 56 micrometers, or less than 54 micrometers, or less than 52 micrometers. In some embodiments, the imaging system has a point spread function having a full width at 15% of maximum greater than 35 micrometers, or greater than 40 micrometers. In some embodiments, the imaging system has a point spread function having a full width at 30% of maximum less than 21 micrometers, or less than 20 micrometers, or less than 19.5 micrometers, or less than 19.1 micrometers. In some embodiments, the imaging system has a point spread function having a full width at 30% of maximum greater than 10 micrometers or greater than 15 micrometers.

In some embodiments, the optical film 800 substantially transmits light having a first polarization state and substantially reflects light having an orthogonal second polarization state in a predetermined wavelength range extending from 400 nm to 700 nm, or from 430 nm to 630 nm, or from 450 nm to 1000 nm. Substantially reflecting can be understood to mean a reflectivity of at least 60% and substantially transmitting can be understood to mean a transmittance of at least 60%. The light having the first polarization state may be p-polarized light having the electric field in the z′ direction and the light having the second polarization state may be s-polarized light having the electric field in the y′ direction. In some embodiments, the polarizer 881 is oriented so that the collimated light from collimating lens 883 has the second polarization state. In some embodiments, optical lens 885 has an f-number in a range of 1.8 to 2.2, or 1.9 to 2.1, or 2.0 to 2.1. In some embodiments, optical lens 885 is centered on optical axis 855 which makes an angle of about 30 to 60 degrees, or about 35 to 55 degrees, or about 40 to 50 degrees with the optical film 800. In some embodiments, the optical film 800 has a total transmittance T=T1 when the light 876 is first incident on the first major surface (facing first prism 830) of the optical film 800. In some embodiments, the PBS 850 is positioned such that second prism 840 is facing optical lens 885 and the optical film 800 has a total transmittance T=T2 when the light 876 is first incident on the second major surface (facing second prism 840) of the optical film 800. A schematic illustration of the transmittance as a function of wavelength when the collimated light from collimating lens 883 has the second polarization state is illustrated in FIG. 10. The predetermined wavelength range is from λ1 to λ2. In some embodiments, a maximum difference, Δsmax, between T1 and T2 is less than 0.02%, or less than 0.015%, or less than 0.01%, or less than about 0.008%, or less than about 0.006% as a function of a wavelength of the light 876 in the predetermined wavelength range. Δsmax is the maximum over the predetermined wavelength range of |T1−T2|. For example, a maximum difference between T1 and T2 may occur at a wavelength where T1 is 0.01% (or 10⁻⁴) and T2 is 0.006% so that the maximum difference is 0.004%. In some embodiments, T1 and T2 are each less than about 0.05% (or 5×10⁻⁴), or less than about 0.04%, or less than about 0.03%, or less than about 0.02%, or less than about 0.015% for each wavelength in the predetermined wavelength range.

In some embodiments, the polarizer 881 is oriented so that the collimated light from collimating lens 883 has the first polarization state. In such embodiments, the optical film 800 may have a total transmittance T=T3 when the light 876 is first incident on the first major surface of the optical film 800 and a total transmittance T=T4 when the light is first incident on the second major surface of the optical film (e.g., when the PBS 850 is positioned such that second prism 840 is facing optical lens 885). A schematic illustration of the transmittance as a function of wavelength when the collimated light from collimating lens 883 has the first polarization state is illustrated in FIG. 11. In some embodiments, an average difference, Δavg, between T3 and T4 is less than 3%, or less than 2%, or less than 1%, or less than 0.5% as a function of the wavelength of the incident light in the predetermined wavelength range. Δavg is the unweighted average over the predetermined wavelength range of |T3−T4|. In some embodiments, a maximum difference, Δpmax, between T3 and T4 is less than 5%, or less than 4%, or less than 3% as a function of a wavelength of the light 876 in the predetermined wavelength range. Δpmax is the maximum over the predetermined wavelength range of |T3−T4|. In some embodiments, an average over the predetermined wavelength range of each of T3 and T4 is at least 92%, or at least 93%, or at least 94%, or at least 95%.

Terms such as “about” and “substantially” will be understood in the context in which they are used and described in the present description by one of ordinary skill in the art. If the use of “about” as applied to quantities expressing feature sizes, amounts, and physical properties is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, “about” will be understood to mean within 5 percent of the specified value. A quantity given as about a specified value can be precisely the specified value. For example, if it is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, a quantity having a value of about 1, means that the quantity has a value between 0.95 and 1.05, and that the value could be 1. If the use of “substantially normal” is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, “substantially normal” will mean within 30 degrees of normal. Directions described as substantially normal may, in some embodiments, be within 20 degrees, or within 10 degrees of normal, or may be normal or nominally normal.

The following is a list of exemplary embodiments of the present description.

Embodiment 1 is an optical film comprising a plurality of polymeric interference layers, each interference layer reflecting or transmitting light primarily by optical interference for at least one wavelength in a predetermined wavelength range extending at least from 450 nm to 1000 nm, a total number of the interference layers greater than about 100 and less than about 300, such that for light in the predetermined wavelength range and substantially normally incident on the optical film, the plurality of interference layers has an average optical transmittance greater than about 85% for a first polarization state, an average optical reflectance greater than about 95% for an orthogonal second polarization state, and an average optical transmittance less than about 5% for the second polarization state.

Embodiment 2 is the optical film of Embodiment 1 defining an x-axis along the second polarization state, a y-axis along the first polarization state perpendicular to the x-axis, and a z-axis along a thickness direction of the optical film orthogonal to the x- and y-axes, the plurality of polymeric interference layers comprising a plurality of alternating first and second layers, each first and second layer having an index nx along the x-axis, an index ny along the y-axis, and an index nz along the z-axis, for each first layer an absolute value of a difference between ny and nz less than 0.008 and a difference between nx and ny greater than 0.2, for each second layer an absolute value of a difference between ny and nz less than 0.005, and a difference between nx of the first layer and nx of the second layer greater than 0.2.

Embodiment 3 is the optical film of Embodiment 2, wherein for each first layer, 1.8≤nx≤1.9, 1.5≤ny≤1.6, and 1.5≤nz≤1.6, and for each second layer, each of nx, ny and nz is between 1.5 and 1.6.

Embodiment 4 is the optical film of Embodiment 1 further comprising a dichroic polarizer formed integrally with the plurality of polymeric interference layers, wherein the dichroic polarizer transmits at least 80% of light having the first polarization state and absorbs at least 50% of light having the second polarization state.

Embodiment 5 is the optical film of Embodiment 1, wherein a first optical stack comprises the plurality of polymeric interference layers, the optical film further comprising a second optical stack disposed on the first optical stack, the second optical stack comprising a second plurality of polymeric interference layers, each interference layer in the second optical stack reflecting or transmitting light primarily by optical interference for at least one wavelength in the predetermined wavelength range, a total number of the interference layers in the second optical stack greater than about 100 and less than about 300, such that for light in the predetermined wavelength range and substantially normally incident on the optical film, the second plurality of interference layers has an average optical transmittance greater than about 85% for the first polarization state, an average optical reflectance greater than about 95% for the second polarization state, and an average optical transmittance less than about 5% for the second polarization state.

Embodiment 6 is the optical film of Embodiment 5, wherein the first and second optical stacks are spaced apart by one or more spacer layers.

Embodiment 7 is the optical film of Embodiment 6, wherein each of the first and second optical stacks comprises interference layers closer to the one or more spacer layers that reflect longer wavelengths and interference layers farther from the one or more spacer layers that reflect shorter wavelengths.

Embodiment 8 is the optical film of Embodiment 6, wherein the one or more spacer layers comprise a dichroic polarizer transmitting at least 80% of light in the predetermined wavelength range having the first polarization state and absorbing at least 50% of light in the predetermined wavelength range having the second polarization state.

Embodiment 9 is the optical film of Embodiment 5, wherein for a first outermost interference layer being an interference layer in the first optical stack farthest from the second optical stack and a second outermost interference layer being an interference layer in the second optical stack farthest from the first optical stack, the first and second outermost interference layers have optical thicknesses equal to one quarter of respective first and second wavelengths in the predetermined wavelength range, a difference between the first and second wavelengths being less than about 40 nm.

Embodiment 10 is a polarizing beam splitter (PBS) comprising:

a first prism comprising a first hypotenuse; a second prism comprising a second hypotenuse facing the first hypotenuse; and the optical film of any one of Embodiments 1 to 9 disposed between and adhered to the first and second hypotenuses.

Embodiment 11 is the PBS of Embodiment 10, wherein when the PBS is incorporated in an imaging system comprising a point source, a collimating optical lens for collimating light emitted by the point source, and an imaging optical lens having an f-number in a range of 4.5 to 5.5, with the optical film in the PBS receiving light from the imaging optical lens and reflecting the received light toward an image surface, the imaging system has a point spread function having a full width at 20% of maximum less than 33 micrometers or a full width at 15% of maximum less than 60 micrometers.

Embodiment 12 is the PBS of Embodiment 11, wherein the full width at 20% of maximum is less than 33 micrometers and the full width at 15% of maximum is less than 60 micrometers.

Embodiment 13 is the PBS of Embodiment 11, wherein the full width at 20% of maximum is less than 32 micrometers and the full width at 15% of maximum is less than 56 micrometers.

Embodiment 14 is the PBS of Embodiment 10, wherein for a collimated light having the second polarization state and incident on the optical film within the PBS after going through an optical lens having an f-number between 1.8 to 2.2 and centered on an optical axis making an angle of about 30 to 60 degrees with the optical film, the optical film has a total transmittance T1 when the light is first incident on a first major surface of the optical film and a total transmittance T2 when the light is first incident on an opposite second major surface of the optical film, wherein a maximum difference between T1 and T2 is less than 0.02% as a function of a wavelength of the incident light in a wavelength range from 430 to 630 nm.

Embodiment 15 is the PBS of Embodiment 10, wherein for a collimated light having the first polarization state and incident on the optical film within the PBS after going through an optical lens having an f-number between 1.8 to 2.2 and centered on an optical axis making an angle of about 30 to 60 degrees with the optical film, the optical film has a total transmittance T3 when the light is first incident on a first major surface of the optical film and a total transmittance T4 when the light is first incident on an opposite second major surface of the optical film, wherein an average of each of T3 and T4 over a wavelength range from 430 to 630 nm is at least 92%.

Embodiment 16 is an optical film comprising a plurality of stacked first polymeric interference layers disposed on a plurality of stacked second polymeric interference layers, each first and second interference layers reflecting or transmitting light primarily by optical interference for at least one wavelength in a same predetermined wavelength range, an outermost first interference layer being the first interference layer farthest from the plurality of stacked second interference layers, an outermost second interference layer being the second interference layer farthest from the plurality of stacked first interference layers, the outermost first and second interference layers having optical thicknesses equal to one quarter of respective first and second wavelengths in the predetermined wavelength range, a difference between the first and second wavelengths being less than about 40 nm.

Embodiment 17 is the optical film of Embodiment 16, an innermost first interference layer being the first interference layer closest to the plurality of stacked second interference layers, an innermost second interference layer being the second interference layer closest to the plurality of stacked first interference layers, the innermost first and second interference layers having optical thicknesses equal to one quarter of respective third and fourth wavelengths in the predetermined wavelength range, a difference between the third and fourth wavelengths being less than about 80 nm.

Embodiment 18 is the optical film of Embodiment 16, wherein for light in the predetermined wavelength range and substantially normally incident on the optical film, each of the plurality of stacked first polymeric interference layers and the plurality of stacked second polymeric interference layers has an average optical transmittance greater than about 85% for a first polarization state, an average optical reflectance greater than about 95% for an orthogonal second polarization state, and an average optical transmittance less than about 5% for the second polarization state.

Embodiment 19 is the optical film of Embodiment 18 defining an x-axis along the second polarization state, a y-axis along the first polarization state perpendicular to the x-axis, and a z-axis along a thickness direction of the optical film orthogonal to the x- and y-axes, each of the plurality of stacked first polymeric interference layers and the plurality of stacked second interference layers comprising a plurality of alternating first and second layers, each first and second layer having an index nx along the x-axis, an index ny along the y-axis, and an index nz along the z-axis, wherein for each first layer, an absolute value of a difference between ny and nz is less than 0.008 and a difference between nx and ny is greater than 0.2; for each second layer, an absolute value of a difference between ny and nz is less than 0.005; and a difference between nx of the first layer and nx of the second layer is greater than 0.2.

Embodiment 20 is a polarizing beam splitter (PBS) comprising:

a first prism comprising a first hypotenuse; a second prism comprising a second hypotenuse facing the first hypotenuse; and the optical film of any one of Embodiments 16 to 19 disposed between and adhered to the first and second hypotenuses.

Embodiment 21 is the PBS of Embodiment 20, wherein when the PBS is incorporated in an imaging system comprising a point source, a collimating optical lens for collimating light emitted by the point source, and an imaging optical lens having an f-number in a range of 4.5 to 5.5, with the optical film in the PBS receiving light from the imaging optical lens and reflecting the received light toward an image surface, the imaging system has a point spread function having a full width at 20% of maximum less than 33 micrometers or a full width at 15% of maximum less than 60 micrometers.

Embodiment 22 is the PBS of Embodiment 20, wherein for a collimated light having the second polarization state and incident on the optical film within the PBS after going through an optical lens having an f-number between 1.8 to 2.2 and centered on an optical axis making an angle of about 30 to 60 degrees with the optical film, the optical film has a total transmittance T1 when the light is first incident on a first major surface of the optical film and a total transmittance T2 when the light is first incident on an opposite second major surface of the optical film, wherein a maximum difference between T1 and T2 is less than 0.02% as a function of a wavelength of the incident light in the predetermined wavelength range.

Embodiment 23 is the PBS of Embodiment 20, wherein for a collimated light having the first polarization state and incident on the optical film within the PBS after going through an optical lens having an f-number between 1.8 to 2.2 and centered on an optical axis making an angle of about 30 to 60 degrees with the optical film, the optical film has a total transmittance T3 when the light is first incident on a first major surface of the optical film and a total transmittance T4 when the light is first incident on an opposite second major surface of the optical film, wherein an average of each of T3 and T4 over a wavelength range from 430 to 630 nm is at least 92%.

Embodiment 24 is a polarizing beam splitter (PBS) comprising:

a first prism comprising a first hypotenuse; a second prism comprising a second hypotenuse facing the first hypotenuse; and the optical film of any one of Embodiments 16 to 19 disposed between and adhered to the first and second hypotenuses, the optical film further comprising a first optical stack facing the first prism and comprising the plurality of stacked first polymeric interference layers and a second optical stack facing the second prism and comprising plurality of stacked first polymeric interference layers, the first optical stack disposed on, and spaced apart by a light absorbing linear polarizer from, a second optical stack such that when the PBS is incorporated in an imaging system where an image light entering the PBS, exits the PBS after being sequentially reflected by the first optical stack, transmitted by the optical film, and reflected by the second optical stack, the light absorbing linear polarizer absorbs at least 50% of image light scattered by at least one of the first and second optical stacks while absorbing less than 2% of the image light.

Embodiment 25 is an optical film comprising a first optical stack disposed on, and spaced apart by one or more spacer layers from, a second optical stack, each optical stack comprising a plurality of polymeric interference layers reflecting and transmitting light primarily by optical interference in a same predetermined wavelength range extending at least from 450 to 600 nm, such that for light in the predetermined wavelength range and substantially normally incident on the optical film, the plurality of the interference layers in each optical stack transmits at least 80% of light having a first polarization state, reflects at least 90% of light having an orthogonal second polarization state, and transmits less than 5% of light having the second polarization state, each spacer layer in the one or more spacer layers not reflecting or transmitting light primarily by optical interference, each optical stack having interference layers closer to the one or more spacer layers that reflect longer wavelengths and interference layers farther from the one or more spacer layers that reflect shorter wavelengths, wherein the first and second optical stacks and the one or more spacer layers are formed integrally with one another.

Embodiment 26 is the optical film of Embodiment 25, an outermost first interference layer being the polymeric interference layer in the first optical stack farthest from the second optical stack, an outermost second interference layer being the polymeric interference layer in the second optical stack farthest from the first optical stack, the outermost first and second interference layers having optical thicknesses equal to one quarter of respective first and second wavelengths in the predetermined wavelength range, a difference between the first and second wavelengths being less than about 40 nm.

Embodiment 27 is the optical film of Embodiment 25 defining an x-axis along the second polarization state, a y-axis along the first polarization state perpendicular to the x-axis, and a z-axis along a thickness direction of the optical film orthogonal to the x- and y-axes, wherein for each optical stack, the plurality of polymeric interference layers comprise a plurality of alternating first and second layers, each first and second layer having an index nx along the x-axis, an index ny along the y-axis, and an index nz along the z-axis, for each first layer an absolute value of a difference between ny and nz less than 0.008 and a difference between nx and ny greater than 0.2, for each second layer an absolute value of a difference between ny and nz less than 0.005, and a difference between nx of the first layer and nx of the second layer greater than 0.2.

Embodiment 28 is a polarizing beam splitter (PBS) comprising:

a first prism comprising a first hypotenuse; a second prism comprising a second hypotenuse facing the first hypotenuse; and the optical film of any one of Embodiments 25 to 27 disposed between and adhered to the first and second hypotenuses, wherein when the PBS is incorporated in an imaging system comprising a point source, a collimating optical lens for collimating light emitted by the point source, and an imaging optical lens having an f-number in a range of 4.5 to 5.5, with the optical film in the PBS receiving light from the imaging optical lens and reflecting the received light toward an image surface, the imaging system has a point spread function having a full width at 20% of maximum less than 33 micrometers or a full width at 15% of maximum less than 60 micrometers.

Embodiment 29 is a polarizing beam splitter (PBS) comprising:

a first prism comprising a first hypotenuse; a second prism comprising a second hypotenuse facing the first hypotenuse; and the optical film of any one of Embodiments 25 to 27 disposed between and adhered to the first and second hypotenuses, the first optical stack facing the first prism, the second optical stack facing the second prism, the one or more space layers comprising a light absorbing linear polarizer such that when the PBS is incorporated in an imaging system where an image light entering the PBS, exits the PBS after being sequentially reflected by the first optical stack, transmitted by the optical film, and reflected by the second optical stack, the light absorbing linear polarizer absorbs at least 50% of image light scattered by at least one of the first and second optical stacks while absorbing less than 2% of the image light.

Embodiment 30 is an optical film transmitting at least 80% of normally incident light having a first polarization state in a predetermined wavelength range and reflecting at least 95% of normally incident light having an orthogonal second polarization state in the predetermined wavelength range, the optical film comprising a plurality of polymeric layers, each polymeric layer having an average thickness less than about 200 nm, the plurality of polymeric layers comprising first and second polymeric layers as the two polymeric layers in the plurality of polymeric layers farthest apart from each other, the first and second layers having respective first and second thicknesses, a difference between the first and second thicknesses being less than about 10 nm.

Embodiment 31 is the optical film of Embodiment 30 comprising first and second optical stacks spaced apart by one or more spacer layers, the first optical stack comprising a first plurality of layers in the plurality of polymeric layers, the second optical stack comprising a second plurality of layers in the plurality of polymeric layers, the first optical stack comprising the first polymeric layer, the second optical stack comprising the second polymeric layer.

Embodiment 32 is the optical film of Embodiment 31, wherein the one or more spacer layers comprises a dichroic polarizer transmitting at least 80% of light having the first polarization state and absorbing at least 50% of light having the second polarization state.

Embodiment 33 is a polarizing beam splitter (PBS) comprising:

a first prism comprising a first hypotenuse; a second prism comprising a second hypotenuse facing the first hypotenuse; and the optical film of Embodiment 32 disposed between and adhered to the first and second hypotenuses, the first optical stack facing the first prism, the second optical stack facing the second prism, such that when the PBS is incorporated in an imaging system where an image light entering the PBS, exits the PBS after being sequentially reflected by the first optical stack, transmitted by the optical film, and reflected by the second optical stack, the dichroic polarizer absorbs at least 50% of image light scattered by at least one of the first and second optical stacks while absorbing less than 2% of the image light.

Embodiment 34 is the optical film of Embodiment 30 defining an x-axis along the second polarization state, a y-axis along the first polarization state perpendicular to the x-axis, and a z-axis along a thickness direction of the optical film orthogonal to the x- and y-axes, wherein the plurality of polymeric layers comprise a plurality of alternating first and second interference layers, each first and second interference layer having an index nx along the x-axis, an index ny along the y-axis, and an index nz along the z-axis, for each first interference layer an absolute value of a difference between ny and nz is less than 0.008 and a difference between nx and ny greater than 0.2, for each second layer an absolute value of a difference between ny and nz less than 0.005, and a difference between nx of the first layer and nx of the second layer greater than 0.2.

Embodiment 35 is an optical film comprising opposing first and second major surfaces and adjacent non-overlapping first and second optical stacks disposed therebetween, the first optical stack disposed closer to the first major surface and farther from the second major surface and the second optical stack disposed closer to the second major surface and farther from the first major surface, for each optical stack and the major surface closest to the optical stack: the optical stack comprises a plurality of first interference layers numbering between 50 and 300, each first interference layer reflecting or transmitting light primarily by optical interference, each first interference layer closer to the major surface is thinner than each first interference layer farther from the major surface, each first interference layer comprising orthogonal in-plane indices of refraction nx and ny and an index of refraction nz in a thickness direction of the first interference layer, a difference between ny and nz less than 0.008 and a difference between nx and ny greater than 0.2, wherein the first optical stack is integrally formed with the second optical stack.

Embodiment 36 is a polarizing beam splitter (PBS) comprising:

a first prism comprising a first hypotenuse; a second prism comprising a second hypotenuse facing the first hypotenuse; and an optical film disposed between and adhered to the first and second hypotenuses and comprising a plurality of alternating higher index and lower index layers disposed between opposing first and second major surfaces of the optical film, the optical film substantially transmitting light having a first polarization state and substantially reflecting light having an orthogonal second polarization state in a predetermined wavelength range extending from 400 nm to 700 nm, such that for a collimated light having the second polarization state and incident on the optical film within the PBS after going through an optical lens having an f-number between 1.8 to 2.2 and centered on an optical axis making an angle of about 30 to 60 degrees with the optical film, the optical film has a total transmittance T1 when the light is first incident on the first major surface of the optical film and a total transmittance T2 when the light is first incident on the second major surface of the optical film, wherein a maximum difference between T1 and T2 is less than 0.02% as a function of a wavelength of the incident light in the predetermined wavelength range.

Embodiment 37 is the PBS of Embodiment 36, wherein T1 and T2 are each less than about 0.05% for each wavelength in the predetermined wavelength range.

Embodiment 38 is the PBS of Embodiment 36, such that for a collimated light having the first polarization state and incident on the optical film within the PBS after going through an optical lens having an f-number between 1.8 to 2.2 and centered on an optical axis making an angle of about 30 to 60 degrees with the optical film, the optical film has a total transmittance T3 when the light is first incident on the first major surface of the optical film and a total transmittance T4 when the light is first incident on the second major surface of the optical film, wherein an average difference between T3 and T4 in the predetermined wavelength range is less than 3%.

Embodiment 39 is the PBS of Embodiment 36, such that for a collimated light having the first polarization state and incident on the optical film within the PBS after going through an optical lens having an f-number between 1.8 to 2.2 and centered on an optical axis making an angle of about 30 to 60 degrees with the optical film, the optical film has a total transmittance T3 when the light is first incident on the first major surface of the optical film and a total transmittance T4 when the light is first incident on the second major surface of the optical film, wherein a maximum difference between T3 and T4 is less than 5% as a function of the wavelength of the incident light in the predetermined wavelength range.

Embodiment 40 is the PBS of Embodiment 36, wherein for a collimated light having the first polarization state and incident on the optical film within the PBS after going through an optical lens having an f-number between 1.8 to 2.2 and centered on an optical axis making an angle of about 30 to 60 degrees with the optical film, the optical film has a total transmittance T3 when the light is first incident on a first major surface of the optical film and a total transmittance T4 when the light is first incident on an opposite second major surface of the optical film, wherein an average of each of T3 and T4 over a wavelength range from 430 to 630 nm is at least 92%.

Embodiment 41 is the PBS of Embodiment 36, wherein the optical film defines an x-axis along the second polarization state, a y-axis along the first polarization state perpendicular to the x-axis, and a z-axis along a thickness direction of the optical film orthogonal to the x- and y-axes, each of the alternating higher index and lower index layers having an index nx along the x-axis, an index ny along the y-axis, and an index nz along the z-axis, for each higher index layer an absolute value of a difference between ny and nz less than 0.008 and a difference between nx and ny greater than 0.2, for each lower layer an absolute value of a difference between ny and nz less than 0.005, and a difference between nx of the higher index layer and nx of the lower layer greater than 0.2.

Embodiment 42 is a polarizing beam splitter (PBS) comprising:

a first prism comprising a first hypotenuse; a second prism comprising a second hypotenuse facing the first hypotenuse; and an optical film disposed between and adhered to the first and second hypotenuses and comprising a plurality of alternating higher index and lower index layers disposed between opposing first and second major surfaces of the optical film, the optical film substantially transmitting light having a first polarization state and substantially reflecting light having an orthogonal second polarization state in a predetermined wavelength range extending from 400 nm to 700 nm, such that when the PBS is incorporated in an imaging system comprising a point source, a collimating optical lens for collimating light emitted by the point source, and an imaging optical lens having an f-number in a range of 4.5 to 5.5, with the optical film in the PBS receiving light from the imaging optical lens and reflecting the received light toward an image surface, the imaging system has a point spread function having a full width at 20% of maximum less than 33 micrometers.

Embodiment 43 is the PBS of Embodiment 42, wherein the point spread function has a full width at 15% of maximum less than 60 micrometers.

Embodiment 44 is a polarizing beam splitter (PBS) comprising:

a first prism comprising a first hypotenuse; a second prism comprising a second hypotenuse facing the first hypotenuse; and an optical film disposed between and adhered to the first and second hypotenuses and comprising a plurality of alternating higher index and lower index layers disposed between opposing first and second major surfaces of the optical film, the optical film substantially transmitting light having a first polarization state and substantially reflecting light having an orthogonal second polarization state in a predetermined wavelength range extending from 400 nm to 700 nm, such that when the PBS is incorporated in an imaging system comprising a point source, a collimating optical lens for collimating light emitted by the point source, and an imaging optical lens having an f-number in a range of 4.5 to 5.5, with the optical film in the PBS receiving light from the imaging optical lens and reflecting the received light toward an image surface, the imaging system has a point spread function having a full width at a full width at 15% of maximum less than 60 micrometers.

Embodiment 45 is the PBS of Embodiment 44, wherein the point spread function has a full width at 20% of maximum is less than 33 micrometers.

Embodiment 46 is the PBS of any one of Embodiments 42 to 45, wherein the point spread function has a full width at 30% of maximum less than 20 micrometers.

Embodiment 47 is a polarizing beam splitter (PBS) comprising:

a first prism comprising a first hypotenuse; a second prism comprising a second hypotenuse facing the first hypotenuse; and an optical film disposed between and adhered to the first and second hypotenuses and comprising a first optical stack disposed on, and spaced apart by a light absorbing linear polarizer from, a second optical stack, the first optical stack closer to the first hypotenuse and farther from the second hypotenuse, the second optical stack closer to the second hypotenuse and farther from the first hypotenuse, for each optical stack and the hypotenuse closest to the optical stack: the optical stack comprises a plurality of interference layers numbering between 50 and 300 and reflecting and transmitting light primarily by optical interference in a same predetermined wavelength range extending at least from 450 to 600 nm, the interference layers closer to the hypotenuse configured to primarily reflect shorter wavelengths in the predetermined wavelength range and the interference layers farther from the hypotenuse configured to primarily reflect longer wavelengths in the predetermined wavelength range, such that when the PBS is incorporated in an imaging system where an image light entering the PBS, exits the PBS after being sequentially reflected by the first optical stack, transmitted by the optical film, and reflected by the second optical stack, the light absorbing linear polarizer absorbs at least 50% of image light scattered by at least one of the first and second optical stacks while absorbing less than 2% of the image light.

Embodiment 48 is the PBS of Embodiment 47, wherein the plurality of interference layers of the first optical stack comprises a first layer closest to the first hypotenuse and the plurality of interference layers of the second optical stack comprises a second layer closest to the second hypotenuse, the first and second layers having respective first and second thicknesses, a difference between the first and second thicknesses being less than about 10 nm.

Embodiment 49 is a polarizing beam splitter (PBS) comprising:

a first prism comprising a first hypotenuse; a second prism comprising a second hypotenuse facing the first hypotenuse; and an optical film disposed between and adhered to the first and second hypotenuses and comprising a plurality of alternating higher index and lower index layers disposed between opposing first and second major surfaces of the optical film, the optical film substantially transmitting light having a first polarization state and substantially reflecting light having an orthogonal second polarization state in a predetermined wavelength range extending at least from 430 nm to 630 nm, such that for a collimated light having the first polarization state and incident on the optical film within the PBS after going through an optical lens having an f-number between 1.8 to 2.2 and centered on an optical axis making an angle of about 30 to 60 degrees with the optical film, the optical film has a total transmittance T3 when the light is first incident on the first major surface of the optical film and a total transmittance T4 when the light is first incident on the second major surface of the optical film, wherein an average over the predetermined wavelength range of each of T3 and T4 is at least 92%.

Embodiment 50 is the PBS of Embodiment 49, wherein for a collimated light having the second polarization state and incident on the optical film within the PBS after going through the optical lens having an f-number between 1.8 to 2.2 and centered on an optical axis making an angle of about 30 to 60 degrees with the optical film, the optical film has a total transmittance T1 when the light is first incident on the first major surface of the optical film and a total transmittance T2 when the light is first incident on the second major surface of the optical film, wherein a maximum difference between T1 and T2 is less than 0.02% as a function of a wavelength of the incident light in the predetermined wavelength range.

EXAMPLES Example 1

An optical film was made generally as depicted in FIG. 2. The optical film included two packets of optical repeat units (ORUs), where each packet included 138 ORUs. Each ORU included a high index layer of low-melt PEN (a copolymer of 90% PEN and 10% PET and a low index, approximately isotropic layer, consisting of an 85 wt % Polycarbonate/15 wt % PCTg material, blended at 85 wt % with 15 wt % of PETg. The optical film was made by using conventional multilayer optical film processes as described in described in U.S. Pat. Appl. Pub. No. 2007/0047080 (Stover et al). A melt stream having the desired layers was formed and then cast against a casting wheel to form a cast film. The cast film was then uniaxially oriented using a parabolic tenter with a draw ratio of about 6:1 in the transverse direction to provide the optical film. The ORUs were sequentially numbered from 1 to 276 across the optical film. The resulting thickness of the ORUs as a function of the ORU number is shown in FIG. 12. The resulting refractive indices for the high and low index layers are given in Table 1.

TABLE 1 Wavelength High Index Low Index (nm) nx ny nz Isotropic n 633 1.832 1.571 1.563 1.569 550 1.857 1.582 1.574 1.579 430 1.953 1.628 1.620 1.620

The transmission coefficient for normally incident light in air was measured for the pass and block polarization states and is shown in FIG. 21.

A polarizing beam splitter (PBS) was made with the optical film disposed between BK7 glass prisms. The total transmittance as a function of wavelength for light incident on the optical film in the PBS was measured for a collimated light having a specified polarization state and incident on the optical film within the PBS after going through an optical lens having an f-number of 2.04 and centered on an optical axis making an angle of 45 degrees with the optical film. The specified polarization state was either vertically polarized or horizontally polarized and the PBS was positioned so the vertically polarized light was incident on the optical film as s-polarized light, and horizontally polarized light was incident on the optical film as p-polarized light. The pass axis of the optical film was horizontal. The transmittance was determined with the optical film oriented with the wheel side of the film (side that was cast against the casting wheel) towards the light source (denoted WTS) and with the air side of the film (side opposite the wheel side) toward the light source (denoted ATS). FIGS. 13 and 14 show the transmission coefficient (total transmittance) versus wavelength for s-polarized and p-polarized light, respectively, first incident on the optical film from each side of the film.

Comparative Examples C1-C4

Optical films in a PBS were made and measured as in Example 1. The optical film of Comparative Example C1 included two packets where the second packet was flipped relative to the second packet of Example 1 so that the ORUs closest to one major surface were thicker layers reflecting red wavelengths and the ORUs closer to the opposite major surface were thinner layers reflecting blue wavelengths. The optical film of C1 included about 750 interference layers. The optical film of Comparative Example C2 had a single packet with layer thickness monotonically varying across the optical film and having 325 interference layers. The optical film of Comparative Example C3 includes two packets with substantially non-overlapping thickness ranges so that one packet reflected shorter wavelengths and the other packet reflected longer wavelengths. The optical film of C3 included about 750 interference layers. The film of Comparative Example C4 was similar to that of Comparative Example C2 except the total number of interference layers was 275 and a larger refractive index difference was used to obtain a similar reflectivity.

For each of comparative examples C1-C4, the materials and processes used for manufacturing these films were generally as described for Example 1 except for the total number of interference layers, and their configurations, as described above.

FIGS. 15-17 show the total transmittance for s-polarized light first incident on each side of the optical films of Comparative Examples C1-C3, respectively, in a BK7 prism cube determined for a collimated light having an s-polarization state and incident on the optical film within the PBS after going through an optical lens having an f-number of 2.04 and centered on an optical axis making an angle of 45 degrees with the optical film. RTS and BTS denote that the side of the optical film with layers reflecting in the red was facing the light source or that the side of the optical film with layers reflecting in the blue was facing the light source, respectively.

Example 2

An optical film was made and as in Example 1 except that a center spacer layer between the two packets of ORUs included a combination of 4 different polarizing dichroic dyes listed in Table 2.

TABLE 2 PD-335H (Yellow) Dichroic dye Mitsui Fine Chemical, Japan PD-104 (Red) Dichroic dye Mitsui Fine Chemical, Japan PD-325H (Blue) Dichroic dye Mitsui Fine Chemical, Japan PD-318H (Cyan) Dichroic dye Mitsui Fine Chemical, Japan

The dichroic dyes for Example 2, were added to the center spacer layer between the two packets of ORUs, in the combination as shown in Table 3 where the dichroic dyes were master-batched in PEN resin at the indicated pounds per hour (pph).

TABLE 3 Yellow pph Red pph Blue pph Cyan pph LmPEN/PEN pph 0.19 0.36 0.73 0.90 67.84

The transmission coefficient for normally incident light in air was measured for the pass and block polarization states and is shown in FIG. 22.

The film was tested in a PBS as in Example 1. FIGS. 18 and 19 show the transmission coefficient (total transmittance) versus wavelength for s-polarized and p-polarized light, respectively, first incident on the optical film from each side of the film of Example 2.

Example 3

A PBS was made as in Example 1. The PBS was incorporated in an imaging system (see FIG. 8) having a point source (modeled as a pinhole with a 30 micrometer diameter), a collimating optical lens for collimating light emitted by the point source, and an imaging optical lens having an f-number of 5.0, with the optical film in the PBS receiving light from the imaging optical lens and reflecting the received light toward an image surface. The point spread function was determined and is shown in FIG. 20. The full width at half maximum (FWHM), the full width at 30% of maximum (FW30% Max), the full width at 20% of maximum (FW20% Max), and the full width at 20% of maximum (FW20% Max) were determined and are reported in Table 4. The measurements were repeated using the optical films of Comparative Examples C3 and C4 in place of the optical film of Example 1.

TABLE 4 FWHM FW30% Max FW20% Max FW15% Max (μm) (μm) (μm) (μm) C4 11.4 19.5 35.3 64 C3 8.0 23.1 49.7 80 Ex1 12.0 18.9 29.8 50

Descriptions for elements in figures should be understood to apply equally to corresponding elements in other figures, unless indicated otherwise. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations can be substituted for the specific embodiments shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this disclosure be limited only by the claims and the equivalents thereof.

All references, patents, and patent applications referenced in the foregoing are herein incorporated by reference in their entirety in a consistent manner. In the event of inconsistencies or contradictions between portions of the incorporated references and this application, the information in the preceding description shall control. 

1-15. (canceled)
 16. An optical film comprising a first optical stack disposed on, and spaced apart by one or more spacer layers from, a second optical stack, each optical stack comprising a plurality of polymeric interference layers reflecting and transmitting light primarily by optical interference in a same predetermined wavelength range extending at least from 450 to 600 nm, such that for light in the predetermined wavelength range and substantially normally incident on the optical film, the plurality of interference layers in each optical stack transmits at least 80% of light having a first polarization state, reflects at least 90% of light having an orthogonal second polarization state, and transmits less than 5% of light having the second polarization state, each spacer layer in the one or more spacer layers not reflecting or transmitting light primarily by optical interference, each optical stack having interference layers closer to the one or more spacer layers that reflect longer wavelengths and interference layers farther from the one or more spacer layers that reflect shorter wavelengths, wherein the first and second optical stacks and the one or more spacer layers are formed integrally with one another.
 17. The optical film of claim 16, an outermost first interference layer being the polymeric interference layer in the first optical stack farthest from the second optical stack, an outermost second interference layer being the polymeric interference layer in the second optical stack farthest from the first optical stack, the outermost first and second interference layers having respective first and second thicknesses, a difference between the first and second thicknesses being less than about 10 nm.
 18. The optical film of claim 16, an outermost first interference layer being the polymeric interference layer in the first optical stack farthest from the second optical stack, an outermost second interference layer being the polymeric interference layer in the second optical stack farthest from the first optical stack, the outermost first and second interference layers having optical thicknesses equal to one quarter of respective first and second wavelengths in the predetermined wavelength range, a difference between the first and second wavelengths being less than about 40 nm.
 19. A polarizing beam splitter (PBS) comprising: a first prism comprising a first hypotenuse; a second prism comprising a second hypotenuse facing the first hypotenuse; and the optical film of claim 16 disposed between and adhered to the first and second hypotenuses, wherein when the PBS is incorporated in an imaging system comprising a point source, a collimating optical lens for collimating light emitted by the point source, and an imaging optical lens having an f-number in a range of 4.5 to 5.5, with the optical film in the PBS receiving light from the imaging optical lens and reflecting the received light toward an image surface, the imaging system has a point spread function having a full width at 20% of maximum less than 33 micrometers or a full width at 15% of maximum less than 60 micrometers.
 20. A polarizing beam splitter (PBS) comprising: a first prism comprising a first hypotenuse; a second prism comprising a second hypotenuse facing the first hypotenuse; and the optical film of claim 16 disposed between and adhered to the first and second hypotenuses, the first optical stack facing the first prism, the second optical stack facing the second prism, the one or more space layers comprising a light absorbing linear polarizer such that when the PBS is incorporated in an imaging system where an image light entering the PBS, exits the PBS after being sequentially reflected by the first optical stack, transmitted by the optical film, and reflected by the second optical stack, the light absorbing linear polarizer absorbs at least 50% of image light scattered by at least one of the first and second optical stacks while absorbing less than 2% of the image light.
 21. The optical film of claim 16 defining an x-axis along the second polarization state, a y-axis along the first polarization state perpendicular to the x-axis, and a z-axis along a thickness direction of the optical film orthogonal to the x- and y-axes, wherein for each optical stack, the plurality of polymeric interference layers comprises a plurality of alternating first and second layers, each first and second layer having an index nx along the x-axis, an index ny along the y-axis, and an index nz along the z-axis, for each first layer an absolute value of a difference between ny and nz less than 0.008 and a difference between nx and ny greater than 0.2, for each second layer an absolute value of a difference between ny and nz less than 0.005, and a difference between nx of the first layer and nx of the second layer greater than 0.2.
 22. The optical film of claim 21, wherein for each first layer, 1.8≤nx≤1.9, 1.5≤ny≤1.6, and 1.5≤nz≤1.6, and for each second layer, each of nx, ny and nz is between 1.5 and 1.6.
 23. An optical film transmitting at least 80% of normally incident light having a first polarization state in a predetermined wavelength range and reflecting at least 95% of normally incident light having an orthogonal second polarization state in the predetermined wavelength range, the optical film comprising a plurality of polymeric layers, each polymeric layer having an average thickness less than about 200 nm, the plurality of polymeric layers comprising first and second polymeric layers as the two polymeric layers in the plurality of polymeric layers farthest apart from each other, the first and second layers having respective first and second thicknesses, a difference between the first and second thicknesses being less than about 10 nm.
 24. The optical film of claim 23 defining an x-axis along the second polarization state, a y-axis along the first polarization state perpendicular to the x-axis, and a z-axis along a thickness direction of the optical film orthogonal to the x- and y-axes, wherein the plurality of polymeric layers comprise a plurality of alternating first and second interference layers, each first and second interference layer having an index nx along the x-axis, an index ny along the y-axis, and an index nz along the z-axis, for each first layer an absolute value of a difference between ny and nz less than 0.008 and a difference between nx and ny greater than 0.2, for each second layer an absolute value of a difference between ny and nz less than 0.005, and a difference between nx of the first layer and nx of the second layer greater than 0.2.
 25. The optical film of claim 24, wherein for each first layer, 1.8≤nx≤1.9, 1.5≤ny≤1.6, and 1.5≤nz≤1.6, and for each second layer, each of nx, ny and nz is between 1.5 and 1.6.
 26. The optical film of claim 23 comprising first and second optical stacks spaced apart by one or more spacer layers, the first optical stack comprising a first plurality of layers in the plurality of polymeric layers, the second optical stack comprising a second plurality of layers in the plurality of polymeric layers, the first optical stack comprising the first polymeric layer, the second optical stack comprising the second polymeric layer.
 27. The optical film of claim 26, wherein the one or more spacer layers comprises a dichroic polarizer transmitting at least 80% of light having the first polarization state and absorbing at least 50% of light having the second polarization state.
 28. A polarizing beam splitter (PBS) comprising: a first prism comprising a first hypotenuse; a second prism comprising a second hypotenuse facing the first hypotenuse; and the optical film of claim 27 disposed between and adhered to the first and second hypotenuses, the first optical stack facing the first prism, the second optical stack facing the second prism, such that when the PBS is incorporated in an imaging system where an image light entering the PBS, exits the PBS after being sequentially reflected by the first optical stack, transmitted by the optical film, and reflected by the second optical stack, the dichroic polarizer absorbs at least 50% of image light scattered by at least one of the first and second optical stacks while absorbing less than 2% of the image light.
 29. An optical film comprising a plurality of stacked first polymeric interference layers disposed on a plurality of stacked second polymeric interference layers, each first and second interference layers reflecting or transmitting light primarily by optical interference for at least one wavelength in a same predetermined wavelength range, an outermost first interference layer being the first interference layer farthest from the plurality of stacked second interference layers, an outermost second interference layer being the second interference layer farthest from the plurality of stacked first interference layers, the outermost first and second interference layers having optical thicknesses equal to one quarter of respective first and second wavelengths in the predetermined wavelength range, a difference between the first and second wavelengths being less than about 40 nm.
 30. A polarizing beam splitter (PBS) comprising: a first prism comprising a first hypotenuse; a second prism comprising a second hypotenuse facing the first hypotenuse; and the optical film of claim 29 disposed between and adhered to the first and second hypotenuses, the optical film further comprising a first optical stack facing the first prism and comprising the plurality of stacked first polymeric interference layers and a second optical stack facing the second prism and comprising the plurality of stacked first polymeric interference layers, the first optical stack disposed on, and spaced apart by a light absorbing linear polarizer from, a second optical stack such that when the PBS is incorporated in an imaging system where an image light entering the PBS, exits the PBS after being sequentially reflected by the first optical stack, transmitted by the optical film, and reflected by the second optical stack, the light absorbing linear polarizer absorbs at least 50% of image light scattered by at least one of the first and second optical stacks while absorbing less than 2% of the image light.
 31. The optical film of claim 29, an innermost first interference layer being the first interference layer closest to the plurality of stacked second interference layers, an innermost second interference layer being the second interference layer closest to the plurality of stacked first interference layers, the innermost first and second interference layers having optical thicknesses equal to one quarter of respective third and fourth wavelengths in the predetermined wavelength range, a difference between the third and fourth wavelengths being less than about 80 nm.
 32. The optical film of claim 29 defining an x-axis along the second polarization state, a y-axis along the first polarization state perpendicular to the x-axis, and a z-axis along a thickness direction of the optical film orthogonal to the x- and y-axes, each of the plurality of stacked first polymeric interference layers and the plurality of stacked second interference layers comprising a plurality of alternating first and second layers, each first and second layer having an index nx along the x-axis, an index ny along the y-axis, and an index nz along the z-axis, for each first layer an absolute value of a difference between ny and nz less than 0.008 and a difference between nx and ny greater than 0.2, for each second layer an absolute value of a difference between ny and nz less than 0.005, and a difference between nx of the first layer and nx of the second layer greater than 0.2.
 33. The optical film of claim 32, wherein for each first layer, 1.8≤nx≤1.9, 1.5≤ny≤1.6, and 1.5≤nz≤1.6, and for each second layer, each of nx, ny and nz is between 1.5 and 1.6. 